Position detection apparatuses that detect the position of a marker inserted into a body cavity by causing the marker to produce an alternating magnetic field by means of external power supply and then detecting, outside the body, the alternating magnetic field produced by the marker are well known (e.g., refer to Patent Document 1).
Position detection systems for capsule medical devices that detect the position and the orientation of a capsule medical device delivered into the body of a subject by externally producing a position-detecting magnetic field and detecting the absolute-value intensity of an induced magnetic field produced in a magnetic induction coil disposed in the capsule medical device are also well known (e.g., refer to Non-patent Document 1).
Patent Document 1:    Japanese Unexamined Patent Application, Publication No. 2000-81303
Non-patent Document 1:    Tokunaga plus 7 other authors. Precision Position-detecting System Using an LC Resonant Magnetic Marker. Journal of the Magnetics Society of Japan 2005; Vol. 29, No. 2:153-156